This invention relates to miniature telescoping spring loaded electrical probes (hereinafter telescoping spring probes) for establishing low resistance electrical test connections with conductors on, for example, printed circuit boards.
Telescoping spring probes are generally known for making electrical test connections with conductors on printed circuit boards. They are generally affixed in a jig employed in a circuit tester and continuity or lack of continuity between various electrical nodes on a printed circuit board are detected through the probe.
One type of prior art telescoping spring probe manufactured by the assignee of the present invention has an elongated conductive tubular housing with an interior wall. An elongated electrically conductive plunger telescopes with respect to the housing, being slidably mounted on the interior wall of the housing. The plunger has a conical shaped end at the interior of the housing and a coil compression spring engages the conical shaped end, normally urging the plunger and housing in opposite directions to a fully extended stop. The housing is inserted in a conductive receptacle making electrical contact therewith. An electrical connection is made to the receptacle such as by wire wrap or other conventional means, and a probe on an exposed end of the plunger makes electrical contact with the node of the circuit to be tested.
Modern circuit testers generally require telescoping spring probes for making electrical contact through each probe to a different one of a number of very closely spaced nodes. Typically, it is necessary to make electrical contact with, for example, hundreds of nodes, any one of which may be spaced as closely as 0.100 inch on center. As a result a need has arisen for spring probes with an outside housing (O.D.) diameter in the order of 0.04 to 0.150 inch and an inside housing diameter in the order of 0.025 to 0.110 inch.
In addition it is extremely important that consistent low electrical resistance be maintained between the plunger and the housing. It has now become necessary to ensure such low level electrical resistance, in the order of 10 to 25 milliohms, over hundreds of thousands of telescoping cycles. Such consistent low electrical resistance is required during the entire extent of travel of each individual telescoping cycle of the probe and housing as well as from one cycle to another.
Additionally, many of such very small sized, constantly low electrical resistance telescoping spring probes are required in any one test apparatus which necessitates that the cost of each probe be minimized.
Electrical connection is made between the plunger and housing, in the aforementioned prior art probe, through the coil compression spring as well as between the sliding surfaces of the plunger and the housing. It has been found that the electrical resistance between the plunger and housing varies with prolonged cycling, due to wear of the parts, wear particle buildup between parts, movement between parts, and changes in pressure of the coil compression spring during a telescoping cycle.
Summarizing, there is a need for a telescoping spring probe requiring the combination of miniature size of 0.04 to 0.150 O.D.; consistency of low level electrical resistance between parts in the milliohm range within each telescoping cycle and from cycle to cycle over a high number of telescoping cycles in the order of hundreds of thousands of cycles; and very low cost of manufacture.
Attempts have been made to reduce the electrical resistance between the plunger and housing of telescoping spring probes. In one prior device a telescoping spring loaded probe, of the general type discussed above, is provided. However, one or more small metal balls are positioned between the coil compression spring and the end of the plunger. In one arrangement the end of the plunger is inclined and engages a single ball. In another engagement the end of the plunger is conical shaped with a small ball engaging the inclined conical end of the plunger, and a larger ball is positioned between the small ball and the compression spring. The coil compression spring, in both embodiments, forces a ball tightly against the inside wall of the housing and the ball (or balls) forces the plunger transversely into tight sliding electrical contact with the inside wall of the housing. There are a number of disadvantages with such arrangement, including: excessive wear due to high transverse forces between the inside wall of the housing and the plunger, resulting in low cycle life; wear particles lodge between the plunger and the housing thereby causing varying electrical resistance between the plunger and housing; the contact pressures between the ball, plunger and housing vary over the telescoping cycle due to changes in pressure of the coil spring, causing varying electrical resistance between the housing and plunger; and gold is apparently required on the inside housing wall, increasing costs.